1. Technical Field
The present disclosure relates to underground fluid transfer systems and, in particular, to a manhole base assembly forming a junction between underground pipes and a manhole.
2. Description of the Related Art
Underground pipe systems are used to convey fluids in, e.g., municipal waterworks systems, sewage treatment systems, and the like. In order to provide access to underground piping systems for inspection, maintenance and repair, manholes placed at a street level grade can be opened to reveal manhole risers which descend to a manhole base. The manhole base typically forms a junction between two or more pipes of the underground piping system, as well as the upwardly-extending risers.
Existing manhole base structures are formed as precast cylindrical structures, with additional cylindrical and/or cone shaped risers which may be attached to the manhole base to traverse a vertical distance between the buried manhole base and the street grade above. At street grade, a manhole frame and cover may be used to provide access to the riser structures and manhole base.
In addition to providing access via manholes, manhole bases may be used when a pipeline needs to change direction and/or elevation along its underground run. In this application, the manhole base structure may contain two or more non-coaxial openings for connections to pipes. Seals may be used between the manhole base structure and the adjacent attached pipes to provide fluid-tight seals at the junctions. In order to facilitate flow of fluid between the two pipes through the manhole base structure, interior fluid channels or “inverts” may be provided within the manhole base, extending between the pipe openings.
Existing manhole base structures are cast as relatively large, cylindrical concrete castings. Fluid flow channels may be custom formed using large coring machines to drill holes in the sides of the cast concrete structures at desired locations. Alternatively, the cylindrical concrete castings may be cast using individualized forms for each individual casting configuration. The forms are stripped from the castings after the concrete has set. Because the holes are bored through the cylindrical outer profile of the casting, seals are mounted along the interior perimeter of the holes after the holes are bored. Expansion bands and mechanisms may be used to engage seals in a fluid-tight relationship with the interior surfaces of the bored holes. However, in some cases, such as for very large diameter openings, expansion mechanisms may not be a viable option, particularly due to the cylindrical profile of the outer diameter of the cast manhole base.
Previous efforts have focused on the creation of a manhole base structure which is cast in individualized form sets corresponding to the individual base structure geometry. These individualized form sets provide a non-cylindrical outer surface to the finished casting, and in particular, planar surfaces are provided for the pipe aperture openings into the base structure fluid channel. This arrangement may use pipe seals cast into the concrete material adjacent the pipe aperture, which obviates the need to bore holes in the manhole base after casting, as well as for the use of separate seals and expansion bands typically associated with standard cylindrical manhole base structures as described above. Individualized form sets are not amenable to variable geometry (e.g., elevation and angle) of the pipe apertures, and therefore separate forms are used for each desired geometrical arrangement of the base structure. Thus, individualized form sets associated with such non-cylindrical manhole structures are expensive, numerous to inventory, and not compatible with pre-existing casting equipment.
What is needed is an improvement over the foregoing.